Biology · Principles of Inheritance and Variation · NEET
It has three simple parts. (1) A character is controlled by a pair of tiny units called factors (today we call them alleles). (2) These factors come in pairs, so a normal diploid plant has two factors for each character. (3) When the two factors are different, one factor is dominant and shows in the body, while the other is recessive and stays hidden. So in a monohybrid cross, the F1 shows only one parent's character. This matters for NEET because 2024 asked this exact list of statements.
Pure tall is TT and pure dwarf is tt. Each parent gives one factor to the child, so every F1 plant is Tt. In Tt, the tall factor (T) is dominant and the dwarf factor (t) is recessive. Because dominance means only the dominant factor shows in a heterozygote, all Tt plants look tall. None are dwarf and none are medium height. ReNEET 2026 tested this idea directly as an assertion-reason question.
No. This is a very common mistake. The recessive allele (t) is still fully present inside the Tt plant and stays completely unchanged. It is only hidden, not removed or damaged. The proof is that when Tt is self-crossed, dwarf (tt) plants reappear in the F2 generation. If t had been destroyed, dwarf could never come back. The recessive factor stays pure and separates unchanged into gametes (that is the Law of Segregation).
For most genes, one allele makes a normal working enzyme (or protein). The other allele may be a changed copy that makes a non-working enzyme or no enzyme at all. In a heterozygote, the one normal allele usually makes enough enzyme to give the normal character. So the normal allele looks dominant, and the faulty allele looks recessive because its effect is hidden. This is why NCERT says the unmodified (normal) allele is generally the dominant one.
Tt is a genotype (the actual gene make-up). Its phenotype (the visible character) is tall, because T is dominant. Do not confuse the two. TT and Tt are different genotypes but the SAME phenotype (both tall). Only tt (homozygous recessive) is dwarf. NEET loves to test this, so always write both the genotype and its phenotype when solving crosses.
No, it works for traits with complete dominance only. Some traits break this law. In incomplete dominance (snapdragon flower: red RR x white rr gives pink Rr), the heterozygote is a mix, not one parent's character. In codominance (ABO blood group: IA IB gives AB), both alleles show fully together. NEET often gives snapdragon or blood-group examples to test whether you know these exceptions.
Which one of the following can be explained on the basis of Mendel's law of Dominance? A. Out of one pair of factors one is dominant and the other is recessive. B. Alleles do not show any expression and both the characters appear as such in F2 generation. C. Factors occur in pairs in normal diploid plants. D. The discrete unit controlling a particular character is called factor. E. The expression of only one of the parental characters is found in a monohybrid cross.
Assertion A: In an experiment, Mendel observed that the F1 progeny plants are all tall and none are dwarf. Reason R: Stem height is a contrasting trait, with tall being dominant and dwarf being recessive. Choose the most appropriate answer:
Match the following: (a) Dominance (b) Codominance (c) Pleiotropy (d) Polygenic inheritance with (i) Many genes govern a single character (ii) In a heterozygous organism only one allele expresses itself (iii) In a heterozygous organism both alleles express themselves fully (iv) A single gene influences many characters
Try the real previous-year questions from this chapter — each with the answer and a full solution.
(1) Characters are controlled by discrete units called factors (alleles). (2) Factors occur in pairs. (3) In a pair with two different factors, one is dominant (shows) and the other is recessive (stays hidden).
Tall pea (TT) crossed with dwarf pea (tt) gives an F1 that is all Tt and all tall. The tall factor is dominant, so the dwarf factor is hidden even though it is still present.
The Law of Dominance is usually stated first. The Law of Segregation (purity of gametes) is the second, and the Law of Independent Assortment is the third.
The Law of Dominance explains WHY the F1 shows only one character (the dominant one shows). The Law of Segregation explains HOW the two factors separate into different gametes so hidden characters can reappear in F2.
Incomplete dominance (snapdragon pink flower) and codominance (ABO blood group AB) break complete dominance, because the heterozygote is not just one parent's character.
